Hydraulic tension devices are well known. Common to most devices of this type is an annular body which can be mounted in encircling relation to a bolt which is to be tensioned and which further features an annular piston and cylinder device arranged to exert force on the bolt in an axial direction. For example, EP-A-0129440 and GB-A-991783 referred to therein disclose arrangements in which a hydraulic jacking tool is built into the face of a nut, bolt, or as a tool in its own right.
GB-A-1590131 discloses a bolt tensioning tool of the type in which the piston acts on a separate screw-threaded puller engaged on the threaded portion of a bolt independently of a nut.
Such devices have been used for many years, for example to secure marine propellers and also in the general industrial fastening field. A typical example is a threaded nut having an annular recess machined into one face to constitute the cylinder of a piston and cylinder device. The recess is equipped with an annular rubber tire and an annular piston. In use, the nut is threaded onto a bolt so that the free end of the piston, which is usually in the form of a flange known as the load ring, abuts the hardware from which the bolt projects. Hydraulic fluid is then admitted into the tire via a passage through the nut body and the tire in turn applies thrust to the piston. This thrust is then transmitted directly to the assembly, causing an increase in the axial length of the bolt together with compression of the assembly. As a result, the nut body is urged away from the assembly and the bolt is subjected to tension. Once a suitable gap has opened up, shims are inserted into the space between the underside face of the nut body and the load ring so that the tension in the bolt is maintained after the hydraulic pressure is relieved. Similar results may be obtained by building an annular piston and cylinder device into the head of a conventional bolt. For somewhat lighter duty application, it is possible to replace the tire with conventional piston seals, in the form of rubber rings.
Attempts have been made to use such piston and cylinder devices to tension bolts in applications where relatively high temperatures are experienced. For example there are applications in land based gas and steam turbines and in the nuclear industry. However at the operating temperatures concerned, which may be in excess of 250.degree. C., the use of conventional rubber seals and rubber tires is impractical. Prolonged exposure to temperatures above around 150.degree. C. cause rapid deterioration of conventional rubbers. Accordingly, in an attempt to overcome this problem, it has been proposed to use a metal tire. Whilst this solution is susceptible of use at temperatures as high as 500.degree. C., the metal tire is an expensive item requiring special techniques for its manufacture.